Delay "Tuples are not allowed" until emit time

[simoncozens]

Typeshare can't handle Rust types which contain a tuple. This is annoying, because my Rust types do contain a tuple! But it's even more annoying because I'm planning to tell typeshare explicitly what those types should become in the target language using the type= decorator - so it doesn't even matter that my types contain tuples, because they're immediately overridden and Typeshare doesn't even need to care about them.

This PR moves the parse-time failure to emit time, by storing the parse failure inside a RustType::Unrepresentable variant. If this RustType has a type override, then there's no problem; if there are no type overrides, the unrepresentable type can't be emitted, and then there's a problem.

So:

struct Foo {
    #[typeshare(ypescript(type = "Array<[number, number]>"))]
    pub vec_tuple: Option<Vec<(i32, i32)>>
}

$ typeshare src --lang=typescript --output-file=foo.ts

[2025-12-12 15:45:05.594187 +00:00] INFO [cli/src/main.rs:78] typeshare started generating types
[2025-12-12 15:45:05.594317 +00:00] INFO [cli/src/main.rs:85] Using directories: ["src"]
[2025-12-12 15:45:05.621167 +00:00] INFO [cli/src/main.rs:141] typeshare started writing 1 generated types
[2025-12-12 15:45:05.621732 +00:00] INFO [cli/src/main.rs:148] typeshare finished generating types

$ typeshare src --lang=python --output-file=foo.py

[2025-12-12 15:45:23.641500 +00:00] INFO [cli/src/main.rs:78] typeshare started generating types
[2025-12-12 15:45:23.641712 +00:00] INFO [cli/src/main.rs:85] Using directories: ["src"]
[2025-12-12 15:45:23.669306 +00:00] INFO [cli/src/main.rs:141] typeshare started writing 1 generated types
[2025-12-12 15:45:23.670078 +00:00] ERROR [cli/src/main.rs:72] typeshare failed to generate types: Tuples are not allowed in typeshare types at line 13, column 30
Error: Tuples are not allowed in typeshare types at line 13, column 30